Hydrogen gas (dihydrogen or molecular hydrogen)[12] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.[13]The enthalpy of combustion for hydrogen is −286 kJ/mol:[14]

Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures spontaneously explode by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).[15] Pure hydrogen-oxygen flames emitultraviolet light and are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of aSpace Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of a rich mixture of hydrogen to oxygen which produces a visible flame.[citation needed]